This invention relates to an image forming apparatus such as a color printer, a color digital copy machine, etc., and more particularly to an image forming apparatus for forming each of images of basic color components on a corresponding one of a plurality of photosensitive drums and transferring the images formed on the drums to a single recording paper sheet such that the images overlap each other on the sheet, thereby obtaining a color image.
A conventional color copy machine for forming a color image, which forms toner images of different color components on a plurality of photosensitive drums and sequentially transfers the images to a single recording paper sheet conveyed by a conveyance belt, is known from, for example, Japanese Patent Application KOKAI Publication No. 6-35287.
If in this transfer-type color copy machine, toner images of different color components do not accurately overlap each other on a recording paper sheet P, a displaced color image (in which different color components undesirably overlap) will be obtained. This may be caused by intrinsic inclination or displacement of each image forming section incorporated in the machine, or by wrong image forming timing in each image forming section, or by displacement of the transfer position of the paper sheet P in which each image is transferred.
There is a conventional method for correcting such a displaced image, in which the displacement of an image transferred from each image forming section to the paper sheet P is sensed by, for example, a CCD sensor incorporated in the copy machine, thereby correcting the position of the exposure unit relative to the photosensitive drum or the positional relationship between the image forming sections to correct a blurred image. Since in this method, the positions of images of different color components to be actually transferred to the paper sheet P are sensed and corrected, the displacements of the images can reliably be corrected even where, for example, the conveyance rate of the conveyance belt varies because of its thermal expansion.
However, the CCD sensor is very expensive, and therefore the use of the sensor inevitably increases the cost of the copy machine.
Instead of the above method using the CCD sensor, a method which uses wedge-shaped patterns 261 for displacement correction as shown in FIG. 10 is disclosed by, for example, Japanese Patent Application KOKAI Publication No. 8-278680. Specifically, in this method, the patterns 261 are formed in the areas of the conveyance belt which do not carry the paper sheet P. The patterns 261 are sensed by reflection type or transmission type optical sensors 262, and the displacements of images of different color components are determined on the basis of the sensing results. Each of the patterns 261 consists of wedge pattern components corresponding to color components (Y, M, C, K) and linearly arranged at a predetermined pitch in the sub-scanning direction, i.e. in the conveyance direction of a conveyance belt 251. The wedge-shaped patterns 261 are arranged with a distance therebetween in the main-scanning direction, i.e. in the width direction of the conveyance belt 251.
In this method, the sub-scanning directional displacement can be determined by comparing the time interval at which the first line segments of pattern components which extend in the main-scanning direction are sensed, with a time constant set when the pattern components are formed. Further, the main-scanning directional displacement can be determined by comparing the time periods required from the time points when the first line segments of pattern components are sensed to the time points when the second line segments of the pattern components which extend obliquely from the first line segments are sensed. In other words, if in the FIG. 10 case, the time period required from the time when the first line segment of a pattern component is sensed to the time when the second line segment of the pattern component is sensed is relatively long, it can be said that the pattern component is displaced toward the main-scanning directional center of the conveyance belt 251. On the other hand, if the time period required from the time when the first line segment is sensed to the time when the second line segment is sensed is relatively short, it can be said that the pattern component is displaced toward the main-scanning directional end of the conveyance belt 251.
The above-described method can detect sub-scanning and main-scanning directional displacements of an image and correct the displacements without using an expensive device such as the CCD sensor.
In the above method using the wedge-shaped patterns, however, the main-scanning directional displacement of an image cannot be corrected with high precision, although the sub-scanning directional displacement can be corrected with relatively high precision. This is because the main-scanning directional displacement of a wedge-shaped pattern is determined by comparing the time period required from the time when the first line segment is sensed to the time when the second line segment is sensed, and hence the comparison result may well contain an error due to a change in the running rate of the conveyance belt 251 or in the sensing accuracy of the optical sensors 262.
If the displacement of each image cannot accurately be corrected, images of different color components cannot be made to accurately overlap each other, resulting in a degraded, displaced image.